1. Technical Field
The present invention relates to a single (reflected) or a two port (transmission) distance to fault analyzer configured to measure passive intermodulation (PIM) created by two separate signal sources as well as to measure distance to a fault creating PIM.
2. Related Art
PIM is an unwanted signal or signals generated by the non-linear mixing of two or more frequencies in a passive device such as a connector or cable. PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower using these cellular systems typically have multiple connectors that cause PIMs that can interfere with system operation.
The PIM signals are created when two signals from different systems combine and are then reflected at a PIM point such as a faulty cable connector. If the generated PIM harmonic frequency components fall within the receive band of a base station, it can effectively block a channel and make the base station receiver think that a carrier is present when one is not. Generally the components of concern are 3rd, 5th and 7th order where the third order is of greatest signal strength, and therefore, of primary concern. PIMs can, thus, occur when two base stations operating at different frequencies, such as an AWS device and a PCS device, are in close proximity.
The PIMs can be reduced or eliminated by replacing faulty cables or connectors. Test systems are thus utilized to detect the PIMs enabling a technician to locate the faulty cable or connector. The test system to measure the PIMs, thus, creates signals at two different frequencies, amplifies them, and provides them through cables connecting a base station to antennas on a tower for the base stations. A return signal carrying the PIMs is filtered to select a desired test frequency harmonic where PIMs can be detected, and the PIM and distance to PIM measurement is provided to an operator.
FIG. 1 shows a block diagram of components of a prior art test system setup for measuring PIM. The test system utilizes two signal sources 2 and 4, with a first signal source 2 producing a signal at frequency F1 and the second signal source 4 producing a signal at frequency F2. When these multiple signals are allowed to share the same signal path in a nonlinear transmission medium, the unwanted signals can occur. The 3rd order response is particularly troublesome as it produces signals at 2F1-F2 as well as 2F2-F1.
In the system of FIG. 1, the signal sources 2 and 4 are provided through high power amplifiers (HPAs) 6 and 8 and isolators 10 and 12 to a hybrid combiner 14 to create a combined signal with frequencies F1 and F2 at the hybrid combiner 14 output. The duplexer 16 sends the signals F1 and F2 to the test port P1. A reverse or reflected signal from port P1 is then produced at frequency 2F1-F2, and forwarded through duplexer 16 to switch 18. The switch 18 in the receive (R) position provides the signal 2F1-F2 through an amplifier 20 to a digital receiver or spectrum analyzer 22 for measurement. The port P1 can be connected by cable to port P2 and switch 18 switched over to make a transmission (T) measurement. With the transmission measurement, signals are provided at F1 and F2 with mixing products at 2F1-F2 to port P2. The duplexer 26 provides the signals F1 and F2 to termination 24, while the signal 2F1-F2 is provided from duplexer 26 through switch 18 and amplifier 20 to the digital receiver or spectrum analyzer 22 for measurement.
FIG. 2 shows an example of actual frequencies used when measuring a load with the test system setup of FIG. 1. Components carried over from FIG. 1 to FIG. 2 are similarly labeled, as will be components carried over in subsequent figures. The two signals F1 and F2 and how they create a third interfering signal can be explained using an example measurement setup with two distinct transmitters, a PCS Band transmitter 2 transmitting at F1=1930 MHz and an AWS Band transmitter 4 transmitting at F2=2127.5 MHz. The PIM produced signal, which can be the result of reflection from a corroded connector or antenna in the transmission path, is simulated by PIM source 30 attached to port P1. It is unknown where an actual PIM or multiple PIM sources may be located. This can be especially troubling when multiple connectors are involved as can be present in a PCS/AWS site tower. But, the PIM source 30 in combination with its connecting cable and load can be designed to simulate reflection from at least one connector.
The PIM source 30 generates a signal at 2×1930−2127.5=1732.5 MHz that is in the receive Band of the AWS system 4. A signal is produced at 2F2-F1=2325 MHz, as also shown in FIG. 2, but since that signal is outside the transmit or receive band of either transmitter 2 and 4, it is not relevant to the present measurement. The two signals transmitted from sources 2 and 4 produce about 40 watts of power for each carrier or +46 dBm each. The resulting PIM signal is on the order of −100 dBm.
The receive channel of the AWS source 4 in an actual operating environment can be desensitized by this interfering signal due to the broadband noncorrelated characteristic of the modulation present on both transmit carriers spreading the power over the entire receive channel. DIN 7-16 coax cable connectors typically have PIM values on the order of −140 to −168 dBc. The PIM measurement, thus, must detect signals that are <−146 dBc. Since the desired PIM signal to be measured is the 1732.5 MHz signal, the bandpass filter 32 with center frequency of 1732.5 is used to filter out other signal components and provide the PIM signal for measurement to the digital receiver or spectrum analyzer 22. An exemplary digital receiver or spectrum analyzer 22 used to perform the test can be a Summitek Instruments Model S12000D Passive Intermodulation Analyzer in the D configuration or a Telstra PIMT2V2 low power PIM Tester.
A problem with current test systems is that when an AWS system and a PCS system are in close proximity, a filter is provided in the AWS system to remove PCS components. The band reject filter will eliminate the 1732.5 MHz PIM test signal that is being measured by the system of FIG. 1, as described above, so testing for 3rd order signals cannot be accomplished. Testing for higher order signals such as 5th, 7th or other higher orders will be outside the receive bandwidth of the conventional tester.